A normal ear directs sounds as shown in FIG. 1 from the outer ear pinna 101 through the generally cylindrical ear canal 110 to vibrate the tympanic membrane 102 (eardrum). The tympanic membrane 102 moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the cochlea 104, which in turn functions as a transducer to generate electric pulses to the brain that are interpreted as sounds.
In addition, the inner ear also includes a balance sensing vestibular system which involves the vestibular labyrinth, its three interconnected and mutually orthogonal semi-circular canals: the superior canal 106, posterior canal 107, and horizontal canal 108 (as well as the otolith organs 116 in the utricle and saccule of the inner ear. The canals and otoliths of the vestibular labyrinth contain hair cells 118 in a viscous endolymph 117 to sense head orientation and head movements, thereby activating vestibular nerve fibers 119 that send an electrical balance signal to the brain 105.
In some people, the vestibular system is damaged or impaired. Such vestibular dysfunction can cause balance problems such as unsteadiness, vertigo and unsteady vision. This can be a significant handicap in everyday life. To treat such problems, stimulation of the vestibular system can help to restore the balancing function, and vestibular implants are currently under development to provide such an artificial balance signal.
FIG. 1 also shows some components of a vestibular implant system such as is described in U.S. patent application Ser. No. 61/366,345 (incorporated herein by reference). An external movement signal (from one or more sensors not shown) is processed by an external processor 111 to produce a vestibular stimulation signal. An external transmitter coil 112 couples the stimulation signal through the skin to an implanted receiver coil 113. Implanted vestibular stimulator 114 than delivers the stimulation signal through an electrode lead 109 to vestibular stimulator electrodes 115 that stimulate target neural tissue such as the semicircular canals 106, 107, 108, one or both otolith organs, and/or the vestibular nerve 105 or ganglion for vestibular sensation by the patient as a balance signal.
In animal evaluations of vestibular implant systems, stimulation modulation appears to be effective where the stimulus strength is rate-modulated around a baseline rate and/or amplitude-modulated according to rotational acceleration. But a human patient may experience discomfort (vertigo, etc.) when such an implant initially is powered up and starts to stimulate, as well as when ongoing stimulation stops. It is therefore desirable for the implant power supply to provide stimulation energy that is if possible uninterrupted at all times.
In addition, existing gyro sensors used in vestibular implant systems have relatively high power consumption and require a relatively large battery (either in the implanted part or in an external part of the vestibular implant system) and/or relatively frequent battery re-charging cycles. But again the onset of stimulation (when the vestibular implant is being activated) and switching off (e.g. when the battery is depleted) will be required from time to time, which are challenging situations as the respective changes in stimulation patterns can result in severe discomfort (vertigo etc.). Additionally, in certain situations such a change or loss of stimulation patterns can be possibly dangerous, especially when occurring unexpectedly.